Magnetic tape, magnetic tape cartridge, and magnetic tape apparatus

ABSTRACT

The magnetic tape includes a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, in which an absolute value ΔN of a difference between a refractive index Nxy measured regarding an in-plane direction of the magnetic layer and a refractive index Nz measured regarding a thickness direction of the magnetic layer is 0.25 to 0.40, and a coefficient of friction measured regarding a base portion of a surface of the magnetic layer is equal to or smaller than 0.30, a magnetic tape cartridge and a magnetic tape apparatus including this magnetic tape.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese Patent Application No. 2018-141866 filed on Jul. 27, 2018. The above application is hereby expressly incorporated by reference, in its entirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic tape, a magnetic tape cartridge, and a magnetic tape apparatus.

2. Description of the Related Art

Magnetic recording media are divided into tape-shaped magnetic recording media and disk-shaped magnetic recording media, and tape-shaped magnetic recording media, that is, magnetic tapes are mainly used for storage such as data back-up. As the magnetic tape, a magnetic tape having a configuration in which a magnetic layer including ferromagnetic powder and a binding agent is provided on a non-magnetic support is widely used (for example, see JP2005-243162A).

SUMMARY OF THE INVENTION

A magnetic tape is generally accommodated in a magnetic tape cartridge in a state of being wound around a reel. The recording of information on the magnetic tape and the reproducing thereof are generally performed by setting a magnetic tape cartridge in a magnetic tape apparatus called a drive, and causing the magnetic tape to run in the magnetic tape apparatus, and causing a surface of the magnetic tape (surface of a magnetic layer) and a magnetic head to come into contact with each other for sliding. In a case Where the sliding between a surface of the magnetic tape and a magnetic head is repeated, a frequency of generation of a partial decrease in reproducing signal amplitude (referred to as “missing pulse”) may increase, during the reproducing of information recorded on the magnetic tape. As the generation frequency of the missing pulse increases, an error rate increases and reliability of a magnetic tape is deteriorated.

However, in recent years, the magnetic tape is used in a data center in which a temperature and humidity are controlled.

Meanwhile, in the data center, power saving is required for cost reduction. In order to realize the power saving, it is desirable to further alleviate the controlling conditions of a usage environment of the magnetic tape in the data center than current conditions or make the control unnecessary.

However, it is assumed that, in a case where the controlling conditions of the usage environment are alleviated or the controlling thereof is not performed, the magnetic tape is used in a low temperature and high humidity environment, for example. Therefore, it is desired to provide a magnetic tape in which a generation frequency of missing pulse is low, even in a case where sliding between a surface of the magnetic tape and a magnetic head is repeated in the low temperature and high humidity environment.

One aspect of the invention provides for a magnetic tape in which a generation frequency of missing pulse is low, even in a case where sliding between a surface of the magnetic tape and a magnetic head is repeated in the low temperature and high humidity environment.

According to one aspect of the invention, there is provided a magnetic tape comprising: a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, in which an absolute value ΔN of a difference between a refractive index Nxy measured regarding an in-plane direction of the magnetic layer and a refractive index Nz measured regarding a thickness direction of the magnetic layer (also referred to as “ΔN (of the magnetic layer)”) is 0.25 to 0.40, and a coefficient of friction measured regarding a base portion of a surface of the magnetic layer (also referred to as a “base friction”) is equal to or smaller than 0.30.

In one aspect, the difference (Nxy−Nz) between the refractive index Nxy and the refractive index Nz may be 0.25 to 0.40.

In one aspect, the base friction may be 0.10 to 0.30.

In one aspect, the magnetic tape may further comprise a non-magnetic layer including non-magnetic powder and a binding agent between the non-magnetic support and the magnetic layer.

In one aspect, the magnetic tape may further comprise a back coating layer including non-magnetic powder and a binding agent on a surface of the non-magnetic support opposite to a surface provided with the magnetic layer.

According to another aspect of the invention, there is provided a magnetic tape cartridge comprising: the magnetic tape described above.

According to still another aspect of the invention, there is provided a magnetic tape apparatus comprising: the magnetic tape described above; and a magnetic head.

According to one aspect of the invention, it is possible to provide a magnetic tape in which a generation frequency of a missing pulse is low, even in a case where sliding between a surface of a magnetic tape and a magnetic head is repeated in a low temperature and high humidity environment. In addition, according to the other aspect of the invention, it is possible to provide a magnetic tape cartridge and a magnetic tape apparatus including the magnetic tape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Magnetic Tape

One aspect of the invention relates to a magnetic tape including: a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, in which an absolute value ΔN of a difference between a refractive index Nxy measured regarding an in-plane direction of the magnetic layer and a refractive index Nz measured regarding a thickness direction of the magnetic layer is 0.25 to 0.40, and a coefficient of friction measured regarding a base portion of a surface of the magnetic layer (base friction) is equal to or smaller than 0.30.

Hereinafter, measurement methods of ΔN and the base friction will be described.

In the invention and the specification, the absolute value ΔN of the difference between the refractive index Nxy measured regarding the in-plane direction of the magnetic layer and the refractive index Nz measured regarding the thickness direction of the magnetic layer is a value obtained by the following method.

The refractive index regarding each direction of the magnetic layer is obtained using a double-layer model by spectral ellipsometry. In order to obtain the refractive index of the magnetic layer using the double-layer model by spectral ellipsometry, a value of a refractive index of a portion adjacent to the magnetic layer is used. Hereinafter, an example in a case of obtaining the refractive indexes Nxy and Nz of the magnetic layer of the magnetic tape including a layer configuration in which the non-magnetic layer and the magnetic layer are laminated on the non-magnetic support in this order will be described. However, the magnetic tape according to one aspect of the invention may also be a magnetic tape having a layer configuration in which the magnetic layer is directly laminated on the non-magnetic support without the non-magnetic layer interposed therebetween. Regarding the magnetic tape having such a configuration, the refractive index regarding each direction of the magnetic layer is obtained in the same manner as the following method, using the double-layer model of the magnetic layer and the non-magnetic support. In addition, an incidence angle shown below is an incidence angle in a case where the incidence angle is 0° in a case of vertical incidence.

(1) Preparation of Sample for Measurement

Regarding the magnetic tape including a back coating layer on a surface of a non-magnetic support on a side opposite to the surface provided with a magnetic layer, the measurement is performed after removing the back coating layer of a sample for measurement cut from the magnetic tape. The removal of the back coating layer can be performed by a well-known method of dissolving of the back coating layer using a solvent or the like. As the solvent, for example, methyl ethyl ketone can be used. However, any solvent which can remove the back coating layer may be used. The surface of the non-magnetic support after removing the back coating layer is roughened by a well-known method so that the reflected light on this surface is not detected, in the measurement of ellipsometer. The roughening can be performed by a method of polishing the surface of the non-magnetic support after removing the back coating layer by using sand paper, for example. Regarding the sample for measurement cut out from the magnetic tape not including the back coating layer, the surface of the non-magnetic support on a side opposite to the surface provided with the magnetic layer is roughened.

In addition, in order to measure the refractive index of the non-magnetic layer described below, the magnetic layer is further removed and the surface of the non-magnetic layer is exposed. In order to measure the refractive index of the non-magnetic support described below, the non-magnetic layer is also further removed and the surface of the non-magnetic support on the magnetic layer side is exposed. The removal of each layer can be performed by a well-known method so as described regarding the removal of the back coating layer. A longitudinal direction described below is a direction which was a longitudinal direction of the magnetic tape, in a case where the sample for measurement is included in the magnetic tape before being cut out. This point applies to other directions described below, in the same manner.

(2) Measurement of Refractive Index of Magnetic Layer

By setting the incidence angles as 65°, 70°, and 75°, and irradiating the surface of the magnetic layer in the longitudinal direction with an incidence ray having a beam diameter of 300 μm by using the ellipsometer, Δ (phase difference of s-polarized light and p-polarized light) and ψ (amplitude ratio of s-polarized light and p-polarized light) is measured. The measurement is performed by changing a wavelength of the incidence ray by 1.5 nm in a range of 400 to 700 nm, and a measurement value at each wavelength is obtained.

The refractive index of the magnetic layer at each wavelength is obtained with a double-layer model as described below, by using the measurement values of A and iv of the magnetic layer at each wavelength, the refractive index of the non-magnetic layer in each direction obtained by the following method, and the thickness of the magnetic layer.

The zeroth layer which is a substrate of the double-layer model is set as a non-magnetic layer and the first layer thereof is set as a magnetic layer. The double-layer model is created by assuming that there is no effect of rear surface reflection of the non-magnetic layer, by only considering the reflection of the interfaces of air/magnetic layer and magnetic layer/non-magnetic layer. A refractive index of the first layer which is fit to the obtained measurement value the most is obtained by fitting performed by a least squares method. The refractive index Nx of the magnetic layer in the longitudinal direction and a refractive index Nz₁ of the magnetic layer in the thickness direction measured by emitting the incidence ray in the longitudinal direction are obtained as values at the wavelength of 600 nm obtained from the results of the fitting.

In the same manner as described above, except that the direction of incidence of the incidence ray is set as a width direction of the magnetic tape, a refractive index Ny of the magnetic layer in the width direction and a refractive index Nz₂ of the magnetic layer in the thickness direction measured by emitting the incidence ray in the width direction are obtained as values at the wavelength of 600 nm obtained from the results of the fitting.

The fitting is performed by the following method.

In general, “complex refractive index η=r+iκ”. Here, η is a real number of the refractive index, κ is an extinction coefficient, and i is an imaginary number. In a case where a complex dielectric constant ε=ε1+iε2 (ε1 and ε2 satisfies Kramers-Kronig relation), ε1=η²−κ², and ε2=2ηκ, the complex dielectric constant of Nx satisfies that ε_(x)=ε_(x)1+iε_(x)2, and the complex dielectric constant of Nz₁ satisfies that ε_(z1)=ε₁1+iε₁2, in a case of calculating the Nx and Nz₁.

The Nx is obtained by setting ε_(x)2 as one Gaussian, setting any point, where a peak position is 5.8 to 5.1 eV and σ is 4 to 3.5 eV, as a starting point, setting a parameter to be offset to a dielectric constant beyond a measurement wavelength range (400 to 700 nm), and performing least squares fitting with respect to the measurement value. In the same manner, N_(z)1 is obtained by setting any point of ε_(z1)2, where a peak position is 3.2 to 2.9 eV and σ is 1.5 to 1.2 eV, as a starting point, and setting an offset parameter, and performing least squares fitting with respect to the measurement value. Ny and Nz₂ are also obtained in the same manner. The refractive index Nxy measured regarding the in-plane direction of the magnetic layer is obtained as “Nxy=(Nx+Ny)/2”. The refractive index Nz measured regarding the thickness direction of the magnetic layer is obtained as “Nz=(Nz₁+Nz₂)/2”. From the obtained Nxy and Nz, the absolute value ΔN of difference thereof is obtained.

(3) Measurement of Refractive Index of Non-Magnetic Layer

Refractive indexes of the non-magnetic layer at a wavelength of 600 nm (the refractive index in the longitudinal direction, the refractive index in the width direction, the refractive index in the thickness direction measured by emitting the incidence ray in the longitudinal direction, and the refractive index in the thickness direction measured by emitting the incidence ray in the width direction) are obtained in the same manner as in the method described above, except the following points.

The wavelength of the incidence ray is changed by 1.5 nm in the range of 250 to 700 nm.

By using a double-layer model of a non-magnetic layer and a non-magnetic support, the zeroth layer which is a substrate of the double-layer model is set as the non-magnetic support, and the first layer thereof is set as the non-magnetic layer. The double-layer model is created by assuming that there is no effect of rear surface reflection of the non-magnetic support, by only considering the reflection of the interfaces of air/non-magnetic layer and non-magnetic layer/non-magnetic support.

In the fitting, seven peaks (0.6 eV, 2.3 eV, 2.9 eV, 3.6 eV, 4.6 eV, 5.0 eV, and 6.0 eV) are assumed in the imaginary part (a) of the complex dielectric constant, and the parameter to be offset is set to the dielectric constant beyond the measurement wavelength range (250 to 700 nm),

(4) Measurement of Refractive Index of Non-Magnetic Support

The refractive indexes of the non-magnetic support at a wavelength of 600 nm (refractive index in the longitudinal direction, the refractive index in the width direction, the refractive index in the thickness direction measured by emitting the incidence ray in the longitudinal direction, and the refractive index in the thickness direction measured by emitting the incidence ray in the width direction) used for obtaining the refractive indexes of the non-magnetic layer by the double-layer model are obtained in the same manner as in the method described above for measuring the refractive index of the magnetic layer, except the following points.

A single-layer model with only front surface reflection is used, without using the double-layer model.

The fitting is performed by the Cauchy model (n=A+B/λ², n is a refractive index, A and B are respectively constants determined by fitting, and λ is a wavelength).

In the invention and the specification, the “surface of the magnetic layer” is identical to the surface of the magnetic tape on the magnetic layer side, and the “base portion” is a portion of the surface of the magnetic layer of the magnetic tape specified by the following method.

A surface on which volume of a protruded component and volume of a recess component in a visual field measured by an atomic force microscope (AFM) are identical to each other is determined as a reference surface. A projection having a height equal to or greater than 15 nm from the reference surface is defined as a projection. A portion in which the number of such projections is zero, that is, a portion of the surface of the magnetic layer of the magnetic tape in which a projection having a height equal to or greater than 15 nm from the reference surface is not detected is specified as the base portion.

A coefficient of friction measured regarding the base portion (base friction) is a value measured by the following method.

In the base portion (measured part: length of a magnetic tape in a longitudinal direction of 10 μm), a diamond spherical indenter having a radius of 1 μm is allowed to reciprocate once with a load of 100 μN and a speed of 1 μm/sec to measure a frictional force (horizontal force) and a normal force. The frictional force and the normal force measured here are an arithmetical mean of respective values obtained by continuously measuring frictional forces and normal forces during the one reciprocating operation. The measurement described above can be performed with TI-950 type TRIBOINDENTER manufactured by Hysitron, Inc. A value of a coefficient of friction μ is calculated from an arithmetical mean of the frictional forces and an arithmetical mean of the normal forces measured as described above. The coefficient of friction is a value measured by an equation of F=μN, from the frictional force (horizontal force) F (unit: newton (N)) and the normal force N (unit: newton (N)). The measurement and the calculation of the value of the coefficient of friction μ are performed at three portions of the base portion randomly selected from the surface of the magnetic layer of the magnetic tape, and an arithmetical mean of the three measured values obtained is set as a coefficient of friction measured regarding the base portion (base friction).

According to the magnetic tape, the inventors have surmised as follows regarding a reason for a decrease in the generation frequency of the missing pulse, even in a case where sliding between a surface of the magnetic tape and a magnetic head (hereinafter, also simply referred to as a “head”) is repeated in the low temperature and high humidity environment.

In a case of reproducing information recorded on the magnetic tape, in a case where the surface of the magnetic layer is chipped in the sliding of the surface of the magnetic layer and a head, the generated scraps are attached to the head and a head attached material may be generated. The inventors have surmised regarding the reason for the generation of the missing pulse in the low temperature and high humidity environment, a contact state in a case of the sliding of the surface of the magnetic layer and the head easily becomes unstable due to a tendency of an decrease of sliding properties between the surface of the magnetic layer and the head in the low temperature and high humidity environment, and the reason for the unstable contact state is the generation of the head attached material.

Regarding the above-mentioned point, the inventors have thought that ΔN obtained by the method described above is a value which may be an index of a presence state of the ferromagnetic powder in a surface region of the magnetic layer. This ΔN is surmised as a value which is influenced by the effect of various factors such as a presence state of a binding agent or a density distribution of the ferromagnetic powder, in addition to the alignment state of the ferromagnetic powder in the magnetic layer. In addition, it is thought that the magnetic layer in which the ΔN is set as 0.25 to 0.40 by controlling various factors has a high hardness of the surface of the magnetic layer and the chipping thereof due to the sliding with the head hardly occurs. The inventors have surmised that, this contributes to the prevention of the generation of the head attached material due to the chipping of the surface of the magnetic layer during the sliding with the head in the low temperature and high humidity environment, and as a result, this contributes to a decrease in the generation frequency of the missing pulse in the low temperature and high humidity environment.

In addition, the inventors have surmised regarding the base friction as follows.

In recent years, a technology of including non-magnetic powder in the magnetic layer of the magnetic tape is widely performed, Such non-magnetic powder is generally protruded from the surface of the magnetic layer to form projections, and thereby exhibiting various functions. In general, the coefficient of friction measured regarding the magnetic tape is a coefficient of friction measured in a region including such projections. With respect to this, the base friction is measured in a portion of the surface of the magnetic layer of the magnetic tape in which a projection having a height equal to or greater than 15 nm from the reference surface is not detected, that is, the base portion, as described above. It is considered that the base portion has a low frequency of a contact with the head, in a case where the surface of the magnetic layer and the head slide on each other. However, it is thought that, a high coefficient of friction of the base portion which is in contact with the head, even in a case of a low frequency, disturbs smooth sliding between the base portion and the head. It is surmised that unsmooth sliding between the base portion and the head causes a deterioration in sliding properties between the surface of the magnetic layer and the head, With respect to this, it is thought that the base friction in the magnetic tape equal to or smaller than 0.30 contributes to the smooth sliding between the base portion and the head. The inventors have surmised that this also contributes to a decrease in the generation frequency of missing pulse in the low temperature and high humidity environment.

However, the invention is not limited to the above-mentioned surmise.

Hereinafter, the magnetic tape will be described more specifically. Hereinafter, the generation frequency of the missing pulse in the low temperature and high humidity environment is also simply referred to as the “generation frequency of the missing pulse”.

Magnetic Layer

ΔN of Magnetic Layer

ΔN of the magnetic layer of the magnetic tape is 0.25 to 0.40. As described above, it is surmised that the magnetic layer having ΔN of 0.25 to 0.40 has a high hardness of the surface of the magnetic layer, and the chipping thereof due to the sliding with the head in the low temperature and high humidity environment hardly occurs. Accordingly, it is thought that the chipping of the magnetic layer having ΔN in the range described above hardly occurs on the surface of the magnetic layer during the sliding of the surface of the magnetic layer and the head in the low temperature and high humidity environment. It is surmised that this contributes to a decrease in the generation frequency of the missing pulse in the low temperature and high humidity environment. From a viewpoint of further decreasing the generation frequency of the missing pulse, ΔN is preferably 0.25 to 0.35. A specific aspect of means for adjusting ΔN will be described later.

ΔN is an absolute value of a difference between Nxy and Nz. Nxy is a refractive index measured regarding the in-plane direction of the magnetic layer and Nz is a refractive index measured regarding the thickness direction of the magnetic layer. In one aspect, a relation of Nxy>Nz can be satisfied, and in the other aspect, Nxy<Nz can be satisfied. From a viewpoint of electromagnetic conversion characteristics of the magnetic tape, a relationship of Nxy>Nz is preferable, and therefore, the difference between the Nxy and Nz (Nxy−Nz) is preferably 0.25 to 0.40 and more preferably 0.25 to 0.35.

Various means for adjusting ΔN described above will be described later.

Base Friction

The base friction of the magnetic tape is equal to or smaller than 0.30, and is preferably equal to or smaller than 0.28, and more preferably equal to or smaller than 0.26, from a viewpoint of realizing smoother sliding between the base portion and the head. The base friction can be, for example, equal to or greater than 0.10, equal to or greater than 0.15, or equal to or greater than 0.20. However, from a viewpoint of allowing the smooth sliding between the base portion and the head, a low base friction is preferable, and thus, the base friction may be smaller than the values described above.

In the measurement method of the base friction described above, the reason why a projection having a height equal to or greater than 15 nm from the reference surface is defined as a projection is because, normally, a projection recognized as a projection present on the surface of the magnetic layer is mainly a projection having a height equal to or greater than 15 urn from the reference surface. Such a projection is, for example, formed of non-magnetic powder such as an abrasive on the surface of the magnetic layer. With respect to this, it is considered that more microscopic ruggedness than ruggedness formed by such a projection is present on the surface of the magnetic layer. It is surmised that it is possible to adjust the base friction by controlling a shape of the microscopic ruggedness. As means for realizing the adjustment described above, a method of using two or more kinds of ferromagnetic powders having different average particle sizes as the ferromagnetic powder is used. More specifically, it is thought that, the microscopic ruggedness can be formed on the base portion, in a case where the ferromagnetic powder having a greater average particle size becomes a protruded portion, and it is possible to increase a percentage of the protruded portion present on the base portion by increasing a mixing ratio of the ferromagnetic powder having a greater average particle size (or, conversely, to decrease a percentage of protruded portion present on the base portion by decreasing the mixing ratio). Such means will be described later more specifically.

As other means, a method of forming a magnetic layer with other non-magnetic powder having a greater average particle size than that of ferromagnetic powder, in addition to non-magnetic powder such as an abrasive which can form a projection having a height equal to or greater than 15 nm from the reference surface on the surface of the magnetic layer. More specifically, it is thought that, the microscopic ruggedness can be formed on the base portion, in a case where the other non-magnetic powder becomes a protruded portion, and it is possible to increase a percentage of the protruded portion present on the base portion by increasing a mixing ratio of the non-magnetic powder (or, conversely, to decrease a percentage of protruded portion present on the base portion by decreasing the mixing ratio). Such means will be described later more specifically.

In addition, it is also possible to adjust the base friction by combining the two kinds of means.

However, the adjustment means are merely examples, and it is possible to realize a base friction equal to or smaller than 0.35 by any means capable of adjusting the base friction, and such an aspect is also included in the invention.

Hereinafter, the magnetic layer of the magnetic tape will be described more specifically.

Ferromagnetic Powder

As described above, as means for adjusting the base friction, a method of forming a magnetic layer with two or more kinds of ferromagnetic powders having different average particle sizes as ferromagnetic powder is used. In this case, it is preferable that the ferromagnetic powder having a smaller average particle size is used as ferromagnetic powder used with the largest proportion, among the two or more kinds of ferromagnetic powder, from a viewpoint of improving recording density of the magnetic tape. From this viewpoint, in a case where two or more kinds of ferromagnetic powders having different average particle sizes are used as the ferromagnetic powder of the magnetic layer, the ferromagnetic powder having an average particle size equal to or smaller than 50 nm is preferably used, and the ferromagnetic powder having an average particle size equal to or smaller than 40 nm is more preferably used, as the ferromagnetic powder used with the largest proportion. On the other hand, from a viewpoint of stability of magnetization, the average particle size of the ferromagnetic powder used with the largest proportion is preferably equal to or greater than 5 nm, more preferably equal to or greater than 10 nm, and even more preferably equal to or greater than 15 nm. In a case of using one kind of ferromagnetic powder without using two or more kinds of ferromagnetic powders having different average particle sizes, the average particle size of the ferromagnetic powder used is preferably in the range described above due to the reasons described above.

With respect to this, it is preferable that other ferromagnetic powder used with the ferromagnetic powder used with the largest proportion has a greater average particle size than that of the ferromagnetic powder used with the largest proportion. This may be because the base friction can be decreased due to the protruded portion formed of the ferromagnetic powder having a great average particle size on the base portion. From this viewpoint, a difference between an average particle size of the ferromagnetic powder used with the largest proportion and an average particle size of the ferromagnetic powder used therewith, acquired as “(latter average particle size)−(former average particle size)” is preferably 10 to 80 nm, more preferably 10 to 50 nm, even more preferably 10 to 40 nm, and still preferably 12 to 35 nm. As the ferromagnetic powder used with the ferromagnetic powder used with the largest proportion, it is also possible to use two or more kinds of ferromagnetic powders having different average particle sizes. In this case, it is preferable that an average particle size of at least one of ferromagnetic powder of the two or more kinds of ferromagnetic powders satisfies the difference described above, it is more preferable that average particle sizes of more kinds of ferromagnetic powders satisfy the difference described above, and it is even more preferable that average particle sizes of all of the ferromagnetic powders satisfy the difference described above, with respect to the average particle size of the ferromagnetic powder used with the largest proportion.

Regarding two or more kinds of ferromagnetic powders having different average particle sizes, from a viewpoint of controlling base friction, a mixing ratio of the ferromagnetic powder used with the largest proportion to the other ferromagnetic powder (in a case of using two or more kinds of ferromagnetic powders having different average particle sizes as other ferromagnetic powder, the total thereof) is preferably 90.0:10.0 (former:latter) to 99.9:0.1 and more preferably 95.0:5.0 to 99.5:0.5 based on mass.

Here, the ferromagnetic powders having different average particle sizes indicate the total or a part of a batch of the ferromagnetic powders having different average particle sizes. In a case where particle size distribution based on the number or volume of the ferromagnetic powder included in the magnetic layer of the magnetic tape formed with the ferromagnetic powder having different average particle sizes as described above is measured by a well-known measurement method such as a dynamic light scattering method or a laser diffraction method, an average particle size or a maximum peak in the vicinity thereof of the ferromagnetic powder used with the largest proportion can be nominally confirmed in a particle size distribution curve obtained by the measurement. In addition, an average particle size or a peak in the vicinity thereof of each ferromagnetic powder can be confirmed. Accordingly, in a case where the particle size distribution of the ferromagnetic powder included in the magnetic layer of the magnetic tape formed by using ferromagnetic powder having an average particle size of 5 to 50 nm with the largest proportion, for example, is measured, the maximum peak can be generally confirmed in a range of the particle size of 5 to 50 nm in the particle size distribution curve.

A part of the other ferromagnetic powders described above may be substituted with other non-magnetic powder Which will be described later.

In the invention and the specification, the “ferromagnetic powder” means an aggregate of a plurality of ferromagnetic particles. The “aggregate” is not only limited to an aspect in which particles configuring the aggregate directly come into contact with each other, but also includes an aspect in which a binding agent, an additive, or the like is interposed between the particles. The points described above are also applied to various powders such as non-magnetic powder of the invention and the specification, in the same manner. In the invention and the specification, average particle sizes of various powder such as the ferromagnetic powder and the like are values measured by the following method with a transmission electron microscope, unless otherwise noted.

The powder is imaged at a magnification ratio of 100,000 with a transmission electron microscope, the image is printed on photographic printing paper so as to have the total magnification ratio of 500,000 to obtain an image of particles configuring the powder. A target particle is selected from the obtained image of particles, an outline of the particle is traced with a digitizer, and a size of the particle (primary particle) is measured. The primary particle is an independent particle which is not aggregated.

The measurement described above is performed regarding 500 particles randomly extracted. An arithmetical mean of the particle size of 500 particles obtained as described above is an average particle size of the powder. As the transmission electron microscope, a transmission electron microscope H-9000 manufactured by Hitachi, Ltd. can be used, for example. In addition, the measurement of the particle size can be performed by well-known image analysis software, for example, image analysis software KS-400 manufactured by Carl Zeiss. The average particle size shown in examples which will be described later is a value measured by using transmission electron microscope H-9000 manufactured by Hitachi, Ltd. as the transmission electron microscope, and image analysis software KS-400 manufactured by Carl Zeiss as the image analysis software, unless otherwise noted.

As a method of collecting a sample powder from the magnetic tape in order to measure the particle size, a method disclosed in a paragraph 0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in a case where the shape of the particle observed in the particle image described above is a needle shape, a fusiform shape, or a columnar shape (here, a height is greater than a maximum long diameter of a bottom surface), the size (particle size) of the particles configuring the powder is shown as a length of a long axis configuring the particle, that is, a long axis length, (2) in a case where the shape of the particle is a planar shape or a columnar shape (here, a thickness or a height is smaller than a maximum long diameter of a plate surface or a bottom surface), the particle size is shown as a maximum long diameter of the plate surface or the bottom surface, and (3) in a case where the shape of the particle is a sphere shape, a polyhedron shape, or an unspecified shape, and the long axis configuring the particles cannot be specified from the shape, the particle size is shown as an equivalent circle diameter. The equivalent circle diameter is a value obtained by a circle projection method.

In addition, regarding an average acicular ratio of the powder, a length of a short axis, that is, a short axis length of the particles is measured in the measurement described above, a value of (long axis length/short axis length) of each particle is obtained, and an arithmetical mean of the values obtained regarding 500 particles is calculated. Here, unless otherwise noted, in a case of (1), the short axis length as the definition of the particle size is a length of a short axis configuring the particle, in a case of (2), the short axis length is a thickness or a height, and in a case of (3), the long axis and the short axis are not distinguished, thus, the value of (long axis length/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of the particle is specified, for example, in a case of definition of the particle size (1), the average particle size is an average long axis length, in a case of the definition (2), the average particle size is an average plate diameter. In a case of the definition (3), the average particle size is an average diameter (also referred to as an average particle diameter).

In one aspect, the shape of the ferromagnetic particles configuring the ferromagnetic powder included in the magnetic layer can be a plate shape. Hereinafter, the ferromagnetic powder including the plate-shaped ferromagnetic particles is referred to as a plate-shaped ferromagnetic powder. An average plate ratio of the plate-shaped ferromagnetic powder can be preferably 2.5 to 5.0. The average plate ratio is an arithmetical mean of (maximum long diameter/thickness or height) in a case of the definition (2). As the average plate ratio increases, uniformity of the alignment state of the ferromagnetic particles configuring the plate-shaped ferromagnetic powder tends to easily increase by the alignment process, and the value of ΔN tends to increase.

As a preferred specific example of the ferromagnetic powder, hexagonal ferrite powder can be used. For details of the hexagonal ferrite powder, descriptions disclosed in paragraphs 0012 to 0030 of JP2011-225417A, paragraphs 0134 to 0136 of JP2011-216149A, paragraphs 0013 to 0030 of JP2012-204726A, and paragraphs 0029 to 0084 of JP2015-127985A can be referred to, for example.

As a preferred specific example of the ferromagnetic powder, metal powder can also be used. For details of the metal powder, descriptions disclosed in paragraphs 0137 to 0141 of JP2011-216149A and paragraphs 0009 to 0023 of JP2005-251351A can be referred to, for example.

As a preferred specific example of the ferromagnetic powder, ε-iron oxide powder can also be used. As a manufacturing method of the ε-iron oxide powder, a manufacturing method from a goethite, a reverse micelle method, and the like are known. All of the manufacturing methods are well known. Regarding a method of manufacturing the ε-iron oxide powder in which a part of Fe is substituted with substitutional atoms such as Ga, Co, Ti, Al, or Rh, a description disclosed in J. Jpn. Soc. Powder Metallurgy Vol. 61 Supplement, No. S1, pp. 5280 to 5284, J. Mater. Chem. C, 2013, 1, pp. 5200 to 5206 can be referred, for example. However, the manufacturing method of the ε-iron oxide powder capable of being used as the ferromagnetic powder in the magnetic layer is not limited.

The content (filling percentage) of the ferromagnetic powder of the magnetic layer is preferably 50% to 90% by mass and more preferably 60% to 90% by mass. The components other than the ferromagnetic powder of the magnetic layer are at least a binding agent, and one or more kinds of additives may be randomly included. A high filling percentage of the ferromagnetic powder in the magnetic layer is preferable from a viewpoint of improvement of recording density.

Binding Agent and Curing Agent

The magnetic tape is a coating type magnetic tape and includes a binding agent in the magnetic layer. The binding agent is one or more kinds of resin. The resin may be a homopolymer or a copolymer. As the binding agent included in the magnetic layer, a resin selected from a polyurethane resin, a polyester resin, a polyimide resin, a vinyl chloride resin, an acrylic resin obtained by copolymerizing styrene, acrylonitrile, or methyl methacrylate, a cellulose resin such as nitrocellulose, an epoxy resin, a phenoxy resin, and a polyvinylalkylal resin such as polyvinyl acetal or polyvinyl butyral can be used alone or a plurality of resins can be mixed with each other to be used. Among these, a polyurethane resin, an acrylic resin, a cellulose resin, and a vinyl chloride resin are preferable. These resins can be used as the binding agent even in the non-magnetic layer and/or a back coating layer which will be described later. For the binding agent described above, description disclosed in paragraphs 0029 to 0031 of JP2010-024113A can be referred to. In addition, the binding agent may be a radiation curable resin such as an electron beam curable resin. For the radiation curable resin, a description disclosed in paragraphs 0044 and 0045 of JP2011-048878A can be referred to.

An average molecular weight of the resin used as the binding agent can be, for example, 10,000 to 200,000 as a weight-average molecular weight. The weight-average molecular weight of the invention and the specification is a value obtained by performing polystyrene conversion of a value measured by gel permeation chromatography (GPC). As measurement conditions, the following conditions can be used. The weight-average molecular weight shown in examples which will be described later is a value obtained by performing polystyrene conversion of a value measured under the following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8 mmID (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In one aspect, as the binding agent, a binding agent including an acidic group can be used. The acidic group of the invention and the specification is used as a meaning including a state of a group capable of emitting H⁺ in water or a solvent including water (aqueous solvent) to dissociate anions and salt thereof. Specific examples of the acidic group include a sulfonic acid group, a sulfuric acid group, a carboxy group, a phosphoric acid group, and salt thereof. For example, salt of sulfonic acid group (—SO₃H) is represented by —SO₃M, and M represents a group representing an atom (for example, alkali metal atom or the like) which may be cations in water or in an aqueous solvent. The same applies to aspects of salt of various groups described above. As an example of the binding agent including the acidic group, a resin including at least one kind of acidic group selected from the group consisting of a sulfonic acid group and salt thereof (for example, a polyurethane resin or a vinyl chloride resin) can be used. However, the resin included in the magnetic layer is not limited to these resins. In addition, in the binding agent including the acidic group, a content of the acidic group can be, for example, 20 to 500 eq/ton. eq indicates equivalent and SI unit is a unit not convertible. The content of various functional groups such as the acidic group included in the resin can be obtained by a well-known method in accordance with the kind of the functional group. As the binding agent having a great content of the acidic group is used, the value of ΔN tends to increase. The amount of the binding agent used in a magnetic layer forming composition can be, for example, 1.0 to 30.0 parts by mass, and preferably 1.0 to 20.0 parts by mass with respect to 100.0 parts by mass of the ferromagnetic powder. As the amount of the binding agent used with respect to the ferromagnetic powder increases, the value of ΔN tends to increase.

In addition, a curing agent can also be used together with the resin which can be used as the binding agent. As the curing agent, in one aspect, a thermosetting compound which is a compound in which a curing reaction (crosslinking reaction) proceeds due to heating can be used, and in another aspect, a photocurable compound in which a curing reaction (crosslinking reaction) proceeds due to light irradiation can be used. At least a part of the curing agent is included in the magnetic layer in a state of being reacted (crosslinked) with other components such as the binding agent, by proceeding the curing reaction in the magnetic layer forming step. This point is the same as regarding a layer formed by using a composition, in a case where the composition used for forming the other layer includes the curing agent. The preferred curing agent is a thermosetting compound, polyisocyanate is suitable. For details of the polyisocyanate, descriptions disclosed in paragraphs 0124 and 0125 of JP2011-216149A can be referred to, for example. The amount of the curing agent can be, for example, 0 to 80.0 parts by mass with respect to 100.0 parts by mass of the binding agent in the magnetic layer forming composition, and is preferably 50.0 to 80.0 parts by mass, from a viewpoint of improvement of hardness of the magnetic layer.

Additives

The magnetic layer includes ferromagnetic powder and a binding agent, and may include one or more kinds of additives, if necessary. As the additives, the curing agent described above is used as an example. In addition, examples of the additive included in the magnetic layer include non-magnetic powder, a lubricant, a dispersing agent, a dispersing assistant, an antibacterial agent, an antistatic agent, an antioxidant, and carbon black. As the additives, a commercially available product can be suitably selected and used according to the desired properties. For example, for the lubricant, a description disclosed in paragraphs 0030 to 0033, 0035, and 0036 of JP2016-126817A can be referred to. The non-magnetic layer may include the lubricant. For the lubricant which may be included in the non-magnetic layer, a description disclosed in paragraphs 0030, 0031, 0034, 0035, and 0036 of JP2016-126817A can be referred to. For the dispersing agent, a description disclosed in paragraphs 0061 and 0071 of JP2012-133837A can be referred to. The dispersing agent may be included in the non-magnetic layer. For the dispersing agent which may be included in the non-magnetic layer, a description disclosed in a paragraph 0061 of JP2012-133837A can be referred to.

The magnetic layer preferably includes one kind or two or more kinds of non-magnetic powders. As the non-magnetic powder, non-magnetic powder (hereinafter, referred to as a “projection formation agent”) which can function as a projection formation agent which forms projections suitably protruded from the surface of the magnetic layer can be used. The projection formation agent is a component which can contribute to control of friction properties of the surface of the magnetic layer of the magnetic tape. In addition, the magnetic layer may include non-magnetic powder (hereinafter, referred to as an “abrasive”) which can function as an abrasive. The magnetic layer of the magnetic tape preferably includes at least one of the projection formation agent or the abrasive and more preferably includes both of them.

As the projection formation agent which is one aspect of the non-magnetic filler, various non-magnetic powders normally used as a projection formation agent can be used. These may be powder of an inorganic substance or powder of an organic substance. In one aspect, from a viewpoint of homogenization of friction properties, particle size distribution of the projection formation agent is not polydispersion having a plurality of peaks in the distribution and is preferably monodisperse showing a single peak. From a viewpoint of availability of monodisperse particles, the non-magnetic powder included in the magnetic layer is preferably powder of inorganic substances (inorganic powder). Examples of the inorganic powder include powder of inorganic oxide such as metal oxide, metal carbonate, metal sulfate, metal nitride, metal carbide, and metal sulfide, and powder of inorganic oxide is preferable. The projection formation agent is more preferably colloidal particles and even more preferably inorganic oxide colloidal particles. In addition, from a viewpoint of availability of monodisperse particles, the inorganic oxide colloidal particles are more preferably colloidal silica (silica colloidal particles). In the invention and the specification, the “colloidal particles” are particles which are not precipitated and dispersed to generate a colloidal dispersion, in a case where 1 g of the particles is added to 100 mL of at least one organic solvent of at least methyl ethyl ketone, cyclohexanone, toluene, or ethyl acetate, or a mixed solvent including two or more kinds of the solvent described above at any mixing ratio. The average particle size of the colloidal particles is a value obtained by a method disclosed in a paragraph 0015 of JP2011-048878A as a measurement method of an average particle diameter. In addition, in another aspect, the projection formation agent is preferably carbon black.

An average particle size of the projection formation agent is, for example, 30 to 300 nm and is preferably 40 to 200 nm.

Meanwhile, the abrasive is preferably non-magnetic powder having Mohs hardness exceeding 8 and more preferably non-magnetic powder having Mohs hardness equal to or greater than 9. A maximum value of Mohs hardness is 1.0 of diamond. Specifically, powders of alumina (Al₂O₃), silicon carbide, boron carbide (B₄C), SiO₂, TiC, chromium oxide (Cr₂O₃), cerium oxide, zirconium oxide (ZrO₂), iron oxide, diamond, and the like can be used, and among these, alumina powder such as α-alumina and silicon carbide powder are preferable. Regarding the particle size of the abrasive, a specific surface area which is an index of a particle size is, for example, equal to or greater than 14 m²/g, preferably equal to or greater than 16 m²/g, and more preferably equal to or greater than 18 m²/g. In addition, the specific surface area of the abrasive can be, for example, equal to or smaller than 40 m²/g. The specific surface area is a value obtained by a nitrogen adsorption method (also referred to as a Brunauer-Emmett-Teller (BET) one-point method). Hereinafter, the specific surface area obtained by such a method is also referred to as a BET specific surface area.

In addition, from a viewpoint that the projection formation agent and the abrasive can exhibit each function in more excellent manner, the content of the projection formation agent of the magnetic layer is preferably 1.0 to 4.0 parts by mass and more preferably 1.5 to 3.5 parts by mass with respect to 100.0 parts by mass of the ferromagnetic powder. Meanwhile, the content of the abrasive in the magnetic layer is preferably 1.0 to 20.0 parts by mass, more preferably 3.0 to 15.0 parts by mass, and even more preferably 4.0 to 10.0 parts by mass with respect to 100.0 parts by mass of the ferromagnetic powder.

As an example of the additive which can be used in the magnetic layer including the abrasive, a dispersing agent disclosed in paragraphs 0012 to 0022 of JP2013-131285A can be used as a dispersing agent for improving dispersibility of the abrasive of the magnetic layer forming composition.

As described above, in order to control the base friction to be equal to or smaller than 0.30, other non-magnetic powders can also be used, in addition to the non-magnetic powder described above. For such non-magnetic powder, non-magnetic powder having a. Mohs hardness equal to or smaller than 8 is preferable, and various non-magnetic powders normally used in the non-magnetic layer can be used. Specifically, the non-magnetic layer is as described later. As more preferred non-magnetic powder, red oxide can be used. The Mohs hardness of red oxide is approximately 6.

It is preferable that the other non-magnetic powder described above has an average particle size greater than that of the ferromagnetic powder, in the same manner as the other ferromagnetic powder used with the ferromagnetic powder used with the largest proportion described above. This is because the base friction can decrease due to the protruded portion formed of the other non-magnetic powder on the base portion. From this viewpoint, a difference between an average particle size of the ferromagnetic powder and an average particle size of the other non-magnetic powder used therewith, acquired as “(latter average particle size)−(former average particle size)” is preferably 10 to 80 nm and more preferably 10 to 50 nm. In a case of using two or more kinds of ferromagnetic powders having different average particle sizes as the ferromagnetic powder, the ferromagnetic powder used for calculating the difference between the average particle size thereof and the average particle size of the other non-magnetic powder is a ferromagnetic powder used with the largest proportion among two or more kinds of ferromagnetic powders. As the other non-magnetic powder, it is also possible to use two or more kinds of non-magnetic powders having different average particle sizes. In this case, it is preferable that an average particle size of at least one of non-magnetic powder of the two or more of other non-magnetic powders satisfies the difference described above, it is more preferable that average particle sizes of more kinds of non-magnetic powders satisfy the difference described above, and it is even more preferable that average particle sizes of all of the non-magnetic powders satisfy the difference described above, with respect to the average particle size of the ferromagnetic powder.

From a viewpoint of controlling the base friction, a mixing ratio of the ferromagnetic powder to the other non-magnetic powder (in a case of using two or more kinds of non-magnetic powders having different average particle sizes as other non-magnetic powder, the total thereof) is preferably 90.0:10.0 (former:latter) to 99.9:0.1 and more preferably 95.0:5.0 to 99.5:0.5 based on mass.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic tape may include a magnetic layer directly on the non-magnetic support or may include a non-magnetic layer including the non-magnetic powder and the binding agent between the non-magnetic support and the magnetic layer. The non-magnetic powder used in the non-magnetic layer may be powder of inorganic substances or powder of organic substances. In addition, carbon black and the like can be used. Examples of the inorganic substance include metal, metal oxide, metal carbonate, metal sulfate, metal nitride, metal carbide, and metal sulfide. The non-magnetic powder can be purchased as a commercially available product or can be manufactured by a well-known method. For details thereof, descriptions disclosed in paragraphs 0146 to 0150 of JP2011-216149A can be referred to. For carbon black which can be used in the non-magnetic layer, descriptions disclosed in paragraphs 0040 and 0041 of JP2010-024113A can be referred to. The content (filling percentage) of the non-magnetic powder of the non-magnetic layer is preferably 50% to 90% by mass and more preferably 60% to 90% by mass.

In regards to other details of a binding agent or additives of the non-magnetic layer, the well-known technology regarding the non-magnetic layer can be applied. In addition, in regards to the type and the content of the binding agent, and the type and the content of the additive, for example, the well-known technology regarding the magnetic layer can be applied.

The non-magnetic layer of the magnetic tape also includes a substantially non-magnetic layer including a small amount of ferromagnetic powder as impurities or intentionally, together with the non-magnetic powder. Here, the substantially non-magnetic layer is a layer having a residual magnetic flux density equal to or smaller than 10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m (100 Oe), or a layer having a residual magnetic flux density equal to or smaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m (100 Oe). It is preferable that the non-magnetic layer does not have a residual magnetic flux density and coercivity.

Non-Magnetic Support

Next, the non-magnetic support will be described. As the non-magnetic support (hereinafter, also simply referred to as a “support”), well-known components such as polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamide imide, and aromatic polyamide subjected to biaxial stretching are used. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferable. Corona discharge, plasma treatment, easy-bonding treatment, or heat treatment may be performed with respect to these supports in advance.

Back Coating Layer

The magnetic tape can also include a back coating layer including non-magnetic powder and a binding agent on a surface of the non-magnetic support opposite to the surface provided with the magnetic layer. The back coating layer preferably includes any one or both of carbon black and inorganic powder. In regards to the binding agent included in the back coating layer and various additives which can be randomly included therein, a well-known technology regarding the back coating layer can be applied, and a well-known technology regarding the list of the magnetic layer and/or the non-magnetic layer can also be applied. For example, for the back coating layer, descriptions disclosed in paragraphs 0018 to 0020 of JP2006-331625A and page 4, line 65, to page 5, line 38, of U.S. Pat. No. 7,029,774B can be referred to.

Various Thicknesses

The thicknesses of the non-magnetic support and each layer of the magnetic tape will be described below.

The thickness of the non-magnetic support is, for example, 3.0 to 80.0 μm, preferably 3.0 to 50.0 μm, and more preferably 3.0 to 10.0 μm.

A thickness of the magnetic layer can be optimized according to a saturation magnetization of a magnetic head used, a head gap length, a recording signal band, and the like. The thickness of the magnetic layer is normally 10 nm to 100 nm, and is preferably 20 to 90 nm and more preferably 30 to 70 nm, from a viewpoint of realization of high-density recording. The magnetic layer may be at least one layer, or the magnetic layer can be separated into two or more layers having magnetic properties, and a configuration regarding a well-known multilayered magnetic layer can be applied. A thickness of the magnetic layer which is separated into two or more layers is a total thickness of the layers.

A thickness of the non-magnetic layer is, for example, 0.1 to 1.5 μm and preferably 0.1 to 1.0 μm.

A thickness of the back coating layer is preferably equal to or smaller than 0.9 μm and even more preferably 0.1 to 0.7 μm.

The thicknesses of various layers and the non-magnetic support are obtained by exposing a cross section of the magnetic tape in a thickness direction by a well-known method of ion beams or microtome, and observing the exposed cross section with a scanning transmission electron microscope (STEM). For the specific examples of the measurement method of the thickness, a description disclosed regarding the measurement method of the thickness in examples which will be described later can be referred to.

Manufacturing Step

Preparation of Each Layer Forming Composition

Steps of preparing the composition for forming the magnetic layer, the non-magnetic layer, or the back coating layer generally include at least a kneading step, a dispersing step, and a mixing step provided before and after these steps, if necessary. Each step may be divided into two or more stages. The components used in the preparation of each layer forming composition may be added at an initial stage or in a middle stage of each step. As the solvent, one kind or two or more kinds of various solvents generally used for manufacturing a coating type magnetic recording medium can be used. For the solvent, a description disclosed in a paragraph 0153 of JP2011-216149A can be referred to, for example. In addition, each component may be separately added in two or more steps. For example, the binding agent may be separately added in the kneading step, the dispersing step, and a mixing step for adjusting a viscosity after the dispersion. In order to manufacture the magnetic tape, a well-known manufacturing technology of the related art can be used in various steps. In the kneading step, an open kneader, a continuous kneader, a pressure kneader, or a kneader having a strong kneading force such as an extruder is preferably used. For details of the kneading processes, descriptions disclosed in JP1989-106338A (JP-H01-106338A) and JP1989-079274A (JP-H01-079274A) can be referred to. As a disperser, a well-known disperser can be used. In addition, the ferromagnetic powder and the abrasive can also be dispersed separately. The separate dispersion is specifically a method of preparing a magnetic layer forming composition through a step of mixing an abrasive solution including an abrasive and a solvent (here, ferromagnetic powder is not substantially included) with a magnetic liquid including the ferromagnetic powder, a solvent, and a binding agent. The expression “ferromagnetic powder is not substantially included” means that the ferromagnetic powder is not added as a constituent component of the abrasive solution, and a small amount of the ferromagnetic powder mixed as impurities without any intention is allowed. Regarding ΔN, as a period of the dispersion time of the magnetic liquid increases, the value of ΔN tends to increase. This is thought that, as a period of the dispersion time of the magnetic liquid increases, the dispersibility of the ferromagnetic powder in the coating layer of the magnetic layer forming composition increases, and the uniformity of the alignment state of the ferromagnetic particles configuring the ferromagnetic powder by the alignment process tends to easily increase. In addition, as the period of the dispersion time in a case of mixing and dispersing various components of the non-magnetic layer forming composition increases, the value of ΔN tends to increase. The dispersion time of the magnetic liquid and the dispersion time of the non-magnetic layer forming composition may be set so that ΔN of 0.25 to 0.40 can be realized.

In any stage of preparing each layer forming composition, the filtering may be performed by a well-known method. The filtering can be performed by using a filter, for example. As the filter used in the filtering, a filter having a hole diameter of 0.01 to 3 μm (for example, filter made of glass fiber or filter made of polypropylene) can be used, for example.

Coating Step

The magnetic layer can be formed, for example, by directly applying the magnetic layer forming composition onto the non-magnetic support or performing multilayer coating of the magnetic layer forming composition with the non-magnetic layer forming composition in order or at the same time. The back coating layer can be formed by applying the back coating layer forming composition onto a side of the non-magnetic support opposite to the side provided with the magnetic layer (or magnetic layer is to be provided). In addition, the coating step for forming each layer can be also performed by being divided into two or more steps. For example, in one aspect, the magnetic layer forming composition can be applied in two or more steps. In this case, a drying process may be performed or may not be performed during the coating steps of two stages. In addition, the alignment process may be performed or may not be performed during the coating steps of two stages. For details of the coating for forming each layer, a description disclosed in a paragraph 0066 of JP2010-231843A can be referred to. In addition, for the drying step after applying the each layer forming composition, a well-known technology can be applied. Regarding the magnetic layer forming composition, as a drying temperature of a coating layer which is formed by applying the magnetic layer forming composition (hereinafter, also referred to as a “coating layer of the magnetic layer forming composition” or simply a “coating layer”) decreases, the value of ΔN tends to increase. The drying temperature can be an atmosphere temperature for performing the drying step, for example, and may be set so that ΔN of 0.25 to 0.40 can be realized.

Other Steps

For various other steps for manufacturing the magnetic tape, descriptions disclosed in paragraphs 0067 to 0070 of JP2010-231843A can be referred to.

For example, it is preferable to perform the alignment process with respect to the coating layer of the magnetic layer forming composition while the coating layer is wet. From a viewpoint of ease of exhibiting of ΔN of 0.25 to 0.40, the alignment process is preferably performed by disposing a magnet so that a magnetic field is vertically applied to the surface of the coating layer of the magnetic layer forming composition (that is, homeotropic alignment process). The strength of the magnetic field during the alignment process may be set so that ΔN of 0.25 to 0.40 can be realized. In addition, in a case of performing the coating step of the magnetic layer forming composition by the coating steps of two or more stages, it is preferable to perform the alignment process at least after the final coating step, and it is more preferable to perform the homeotropic alignment process. For example, in a case of forming the magnetic layer by the coating steps of two stages, the drying step is performed without performing the alignment process after the first coating step, and then, the alignment process can be performed with respect to the formed coating layer in the second coating step. For the alignment process, various well-known technologies such as descriptions disclosed in a paragraph 0052 of JP2010-024113A can be used. For example, the homeotropic alignment process can be performed by a well-known method such as a method using a pole opposing magnet. In the alignment zone, a drying speed of the coating layer can be controlled depending on a temperature and an air flow of dry air and/or a transportation speed of the Magnetic tape in the alignment zone. In addition, the coating layer may be preliminarily dried before the transportation to the alignment zone.

In addition, the calender process can be performed in any stage after drying the coating layer of the magnetic layer forming composition. For the conditions of the calender process, a description disclosed in a paragraph 0026 of JP2010-231843A can be referred to. As the calender temperature (surface temperature of the calender roll) increases, the value of ΔN tends to increase. The calender temperature may be set so that ΔN of 0.25 to 0.40 can be realized.

As described above, it is possible to obtain the magnetic tape according to one aspect of the invention. The magnetic tape is normally accommodated in a magnetic tape cartridge and the magnetic tape cartridge is mounted on a magnetic tape apparatus (generally referred to as a “drive”). A servo pattern can also be formed in the magnetic layer of the magnetic tape by a well-known method, in order to allow head tracking servo to be performed in the drive.

According to the magnetic tape, it is possible to decrease the generation frequency of the missing pulse, even in a case where the sliding between the surface of the magnetic tape and the magnetic head is repeated in the low temperature and high humidity environment. In one aspect, a low temperature can be higher than 0° C. and equal to or lower than 15° C., and high humidity can be relative humidity of 70% to 100%.

Magnetic Tape Cartridge

One aspect of the invention relates to a magnetic tape cartridge including the magnetic tape.

In the magnetic tape cartridge, the magnetic tape is generally accommodated in a cartridge main body in a state of being wound around a reel. The reel is rotatably provided in the cartridge main body. As the magnetic tape cartridge, a single reel type magnetic tape cartridge including one reel in a cartridge main body and a twin reel type magnetic tape cartridge including two reels in a cartridge main body are widely used. In a case where the single reel type magnetic tape cartridge is mounted in the magnetic tape apparatus (drive) in order to record and/or reproduce information (magnetic signals) on the magnetic tape, the magnetic tape is drawn from the magnetic tape cartridge and wound around the reel on the drive side. A magnetic head is disposed on a magnetic tape transportation path from the magnetic tape cartridge to a winding reel. Sending and winding of the magnetic tape are performed between a reel (supply reel) on the magnetic tape cartridge side and a reel (winding reel) on the drive side. In the meantime, the magnetic head comes into contact with and slides on the surface of the magnetic layer of the magnetic tape, and accordingly, the recording and/or reproduction of information is performed. With respect to this, in the twin reel type magnetic tape cartridge, both reels of the supply reel and the winding reel are provided in the magnetic tape cartridge. The magnetic tape cartridge may be any of single reel type magnetic tape cartridge and twin reel type magnetic tape cartridge. The magnetic tape cartridge may include the magnetic tape according to one aspect of the invention, and a well-known technology can be applied for other configurations.

Magnetic Tape Apparatus

One aspect of the invention relates to a magnetic tape apparatus including the magnetic tape and a magnetic head.

In the invention and the specification, the “magnetic tape apparatus” means a device capable of performing at least one of the recording of information on the magnetic tape or the reproducing of information recorded on the magnetic tape. Such an apparatus is generally called a drive. The magnetic tape apparatus can be a sliding type magnetic tape apparatus. The sliding type apparatus is an apparatus in which the surface of the magnetic layer comes into contact with and slides on the magnetic head, in a case of performing the recording of information on the magnetic tape and/or reproducing of the recorded information.

The magnetic head included in the magnetic tape apparatus can be a recording head capable of performing the recording of information on the magnetic tape, or can be a reproducing head capable of performing the reproducing of information recorded on the magnetic tape. In addition, in one aspect, the magnetic tape apparatus can include both of a recording head and a reproducing head as separate magnetic heads. In another aspect, the magnetic head included in the magnetic tape can also have a configuration of comprising both of a recording element and a reproducing element in one magnetic head. As the reproducing head, a magnetic head (MR head) including a magnetoresistive (MR) element capable of sensitively reading information recorded on the magnetic tape as a reproducing element is preferable. As the MR head, various well-known MR heads can be used. In addition, the magnetic head which performs the recording of information and/or the reproducing of information may include a servo pattern reading element. Alternatively, as a head other than the magnetic head which performs the recording of information and/or the reproducing of information, a magnetic head (servo head) comprising a servo pattern reading element may be included in the magnetic tape apparatus.

In the magnetic tape apparatus, the recording of information on the magnetic tape and/or the reproducing of information recorded on the magnetic tape can be performed by bringing the surface of the magnetic layer of the magnetic tape into contact with the magnetic head and sliding. The magnetic tape apparatus may include the magnetic tape according to one aspect of the invention and well-known technologies can be applied for other configurations.

EXAMPLES

Hereinafter, the invention will be described with reference to examples. However, the invention is not limited to aspects shown in the examples. “Parts” and “%” in the following description mean “parts by mass” and “% by mass”, unless otherwise noted. In addition, steps and evaluations described below are performed in an environment of an atmosphere temperature of 23° C.±1° C., unless otherwise noted.

Example 1

Preparation of Abrasive Solution

3.0 parts of 2,3-dihydroxynaphthalene (manufactured by Tokyo Chemical Industry Co., Ltd.), 31.3 parts of a 32% solution (solvent is a mixed solvent of methyl ethyl ketone and toluene) of a SO₃Na group-containing polyester polyurethane resin (UR-4800 (SO₃Na group: 0.08 meq/g) manufactured by Toyobo Co., Ltd.), and 570.0 parts of a mixed solvent of methyl ethyl ketone and cyclohexanone (mass ratio of 1:1) as a solvent were mixed with 100.0 parts of alumina powder (HIT-80 manufactured by Sumitomo Chemical Co., Ltd.) having a gelatinization ratio of approximately 65°/s and a Brunauer-Emmett-Teller (BET) specific surface area of 20 m²/g, and dispersed in the presence of zirconia beads by a paint shaker for 5 hours. After the dispersion, the dispersion liquid and the beads were separated by a mesh and an alumina dispersion was obtained.

Preparation of Magnetic Layer Forming Composition

Magnetic Liquid Plate-shaped ferromagnetic hexagonal barium ferrite 100.0 parts powder Ferromagnetic powder (1) and ferromagnetic powder (2) shown in Table 1 were used with the amounts shown in Table 1 SO₃Na group-containing polyurethane resin see Table 1 Weight-average molecular weight: 70,000, SO₃Na group: see Table 1 Cyclohexanone 150.0 parts Methyl ethyl ketone 150.0 parts Abrasive Solution Alumina dispersion prepared as described above 6.0 parts Silica Sol (projection forming agent liquid) Colloidal silica (Average particle size: 100 nm) 2.0 parts Methyl ethyl ketone 1.4 parts Other Components Stearic acid 2.0 parts Butyl stearate 2.0 parts Polyisocyanate (CORONATE (registered trademark) 2.5 parts manufactured by Tosoh Corporation) Finishing Additive Solvent Cyclohexanone 200.0 parts Methyl ethyl ketone 200.0 parts

Preparation Method

The magnetic liquid was prepared by beads-dispersing of various components of the magnetic liquid described above by using beads as the dispersion medium in a batch type vertical sand mill. The bead dispersion was performed using zirconia beads (bead diameter: see Table 1) as the beads for the time shown in Table 1 (magnetic liquid bead dispersion time).

The magnetic liquid obtained as described above, the abrasive solution, silica sol, other components, and a finishing additive solvent were mixed with each other and beads-dispersed for 5 minutes, and the treatment (ultrasonic dispersion) was performed with a batch type ultrasonic device (20 kHz, 300 W) for 0.5 minutes. After that, the obtained mixed solution was filtered by using a filter having a hole diameter of 0.5 μm, and the magnetic layer forming composition was prepared.

Preparation of Non-Magnetic Layer Forming Composition

Each component among various components of the non-magnetic layer forming composition shown below excluding stearic acid, butyl stearate, cyclohexanone, and methyl ethyl ketone was beads-dispersed (dispersion medium: zirconia beads (bead diameter: 0.1 mm), dispersion time: see Table 1) by using a batch type vertical sand mill to obtain a dispersion liquid. After that, the remaining components were added into the obtained dispersion liquid and stirred with a dissolver. Then, the obtained dispersion liquid was filtered with a filter (hole diameter: 0.5 μm) and a non-magnetic layer forming composition was prepared.

Non-magnetic inorganic powder: α-iron oxide 100.0 parts Average particle size (average long axis length): 0.15 μm Average acicular ratio: 7 BET specific surface area: 52 m²/g Carbon black 20.0 parts Average particle size: 20 nm Electron beam curable vinyl chloride copolymer 13.0 parts Electron beam curable polyurethane resin 6.0 parts Stearic acid 1.0 parts Butyl stearate 1.0 parts Cyclohexanone 300.0 parts Methyl ethyl ketone 300.0 parts

Preparation of Back Coating Layer Forming Composition

Each component among various components of the back coating layer forming composition shown below excluding stearic acid, butyl stearate, polyisocyanate, and cyclohexanone was kneaded and diluted by an open kneader, and a mixed solution was obtained. After that, the obtained mixed solution was subjected to a dispersion process of 12 passes, with a transverse beads mill and zirconia beads having a bead diameter of 1.0 mm, by setting a bead filling percentage as 80 volume %, a circumferential speed of rotor distal end as 10 m/sec, and a retention time for 1 pass as 2 minutes, After that, the remaining components were added into the obtained dispersion liquid and stirred with a dissolver. Then, the obtained dispersion liquid was filtered with a filter (hole diameter: 1.0 μm) and a back coating layer forming composition was prepared.

Non-magnetic inorganic powder: α-iron oxide 80.0 parts Average particle size (average long axis length): 0.15 μm Average acicular ratio: 7 BET specific surface area: 52 m²/g Carbon black 20.0 parts Average particle size: 20 nm A vinyl chloride copolymer 13.0 parts A sulfonic acid salt group-containing polyurethane resin 6.0 parts Phenylphosphonic acid 3.0 parts Methyl ethyl ketone 155.0 parts Stearic acid 3.0 parts Butyl stearate 3.0 parts Polyisocyanate 5.0 parts Cyclohexanone 355.0 parts

Manufacturing of Magnetic Tape

The non-magnetic layer forming composition was applied and dried onto a polyethylene naphthalate support, and then, an electron beam was emitted with an energy of 40 kGy at an acceleration voltage of 125 kV, to form a non-magnetic layer.

The magnetic layer forming composition was applied on a surface of the formed non-magnetic layer so that the thickness after the drying becomes 50 nm to form a coating layer. A homeotropic alignment process and a drying process were performed by applying a magnetic field having a strength shown in a column of “formation and alignment of magnetic layer” of Table 1 to the surface of the coating layer in a vertical direction by using a pole opposing magnet in an atmosphere at an atmosphere temperature (magnetic layer drying temperature) shown in Table 1, while this coating layer was wet, and a magnetic layer was formed.

After that, the back coating layer forming composition was applied and dried on a surface of the support on a side opposite to the surface on which the non-magnetic layer and the magnetic layer were formed.

Then, a surface smoothing treatment (calender process) was performed with a calender roll configured of only a metal roll, at a calender process speed of 80 m/min, linear pressure of 300 kg/cm (294 kN/m), and a calender temperature (surface temperature of a calender roll) shown in Table 1.

Then, the thermal treatment was performed in the environment of the atmosphere temperature of 70° C. for 36 hours. After the thermal treatment, the slitting was performed so as to have a width of ½ inches (1 inch is 0.0254 meters), and the surface of the magnetic layer was cleaned with a tape cleaning device in which a nonwoven fabric and a razor blade are attached to a device including a sending and winding device of the slit so as to press the surface of the magnetic layer. After that, a servo pattern was formed on the magnetic layer by a commercially available servo writer.

By doing so, a magnetic tape of Example 1 was manufactured.

Examples 2, 3, and 6 and Comparative Examples 1 to 6

A magnetic tape was manufactured by the same method as that in Example 1, except that various conditions shown in Table 1 were changed as shown in Table 1.

In Table 1, in the comparative examples in which “no alignment process” is shown in the column of “formation and alignment of magnetic layer”, the magnetic tape was manufactured without performing the alignment process regarding the coating layer of the magnetic layer forming composition.

Example 4

After forming the non-magnetic layer, the magnetic layer forming composition was applied on the surface of the non-magnetic layer so that the thickness after drying becomes 25 nm to form a first coating layer. The first coating layer was passed through the atmosphere at the atmosphere temperature shown in Table 1 (magnetic layer drying temperature) without application of a magnetic field to form a first magnetic layer (no alignment process).

After that, the magnetic layer forming composition was applied on the surface of the first magnetic layer so that the thickness after drying becomes 25 nm to form a second coating layer. A homeotropic alignment process and a drying process were performed by applying a magnetic field having a strength shown in the column of “formation and alignment of magnetic layer” of Table 1 to the surface of the second coating layer in a vertical direction by using a pole opposing magnet in an atmosphere at an atmosphere temperature (magnetic layer drying temperature) shown in Table 1, while this second coating layer was wet, and a second magnetic layer was formed.

A magnetic tape was manufactured by the same method as that in Example 1, except that the multilayered magnetic layer was formed as described above.

Example 5

A magnetic tape was manufactured by the same method as that in Example 4, except that the amount of the ferromagnetic powder was changed as that shown in Table 1.

Comparative Example 7

After forming the non-magnetic layer, the magnetic layer forming composition was applied on the surface of the non-magnetic layer so that the thickness after drying becomes 25 nm to form a first coating layer. The homeotropic alignment process and the drying process were performed by applying a magnetic field having a strength shown in the column of “formation and alignment of magnetic layer” of Table 1 to the surface of the first coating layer in a vertical direction by using a pole opposing magnet in an atmosphere at an atmosphere temperature (magnetic layer drying temperature) shown in Table 1, while this first coating layer was wet, and a first magnetic layer was formed.

After that, the magnetic layer forming composition was applied on the surface of the first magnetic layer so that the thickness after drying becomes 25 nm to form a second coating layer. The second coating layer was passed through the atmosphere at the atmosphere temperature shown in. Table 1 (magnetic layer drying temperature) without application of a magnetic field to form a second magnetic layer (no alignment process).

A magnetic tape was manufactured by the same method as that in Comparative Example 2, except that the multilayered magnetic layer was formed as described above.

Comparative Example 8

After forming the non-magnetic layer, the magnetic layer forming composition was applied on the surface of the non-magnetic layer so that the thickness after drying becomes 25 nm to form a first coating layer. The homeotropic alignment process and the drying process were performed by applying a magnetic field having a strength shown in the column of “formation and alignment of magnetic layer” of Table 1 to the surface of the first coating layer in a vertical direction by using a pole opposing magnet in an atmosphere at an atmosphere temperature (magnetic layer drying temperature) shown in Table 1, while this first coating layer was wet, and a first magnetic layer was formed.

After that, the magnetic layer forming composition was applied on the surface of the first magnetic layer so that the thickness after drying becomes 25 nm to form a second coating layer. The second coating layer was passed through the atmosphere at the atmosphere temperature shown in Table 1 (magnetic layer drying temperature) without application of a magnetic field to form a second magnetic layer (no alignment process).

A magnetic tape was manufactured by the same method as that in Comparative Example 6, except that the multilayered magnetic layer was formed as described above, and the amount of the ferromagnetic powder was changed as that in Table 1.

Comparative Example 9

After forming the non-magnetic layer, the magnetic layer forming composition was applied on the surface of the non-magnetic layer so that the thickness after drying becomes 25 nm to form a first coating layer. The first coating layer was passed through the atmosphere at the atmosphere temperature shown in Table 1 (magnetic layer drying temperature) without application of a magnetic field to form a first magnetic layer (no alignment process).

After that, the magnetic layer forming composition was applied on the surface of the first magnetic layer so that the thickness after drying becomes 25 nm to form a second coating layer. The homeotropic alignment process and the drying process were performed by applying a magnetic field having a strength shown in the column of “formation and alignment of magnetic layer” of Table 1 to the surface of the second coating layer in a vertical direction by using a pole opposing magnet in an atmosphere at an atmosphere temperature (magnetic layer drying temperature) shown in Table 1, while this second coating layer was wet, and a second magnetic layer was formed.

A magnetic tape was manufactured by the same method as that in Comparative Example 1, except that the multilayered magnetic layer was formed as described above.

The amount of the ferromagnetic powder shown in Table 1 is a content of each ferromagnetic powder based on mass with respect to 100.0 parts by mass of a total of the ferromagnetic powder. An average particle size of the ferromagnetic powder shown in Table 1 is a value obtained by measuring an average particle size by the method described above. All of the ferromagnetic powder shown in Table 1 is plate-shaped ferromagnetic hexagonal barium ferrite powder having an average plate ratio measured by the method described above of 2.5 to 5.0.

Measurement Method

(1) Base Friction

First, marking was performed on a surface of a magnetic layer which is a measurement target with a laser marker in advance, and an atomic force microscope (AFM) image of a portion separated from the mark by a certain distance (approximately 100 μm) was observed. The observation was performed regarding an area of a visual field of 7 μm×7 μm. At this time, marking was performed on the AFM by changing a cantilever to a hard material (single crystal silicon), so as to easily capture a scanning electron microscope (SEM) image of the same portion as will be described later. All of projections having a height equal to or greater than 15 nm from the reference surface were extracted from the AFM image observed as described above. A portion in which it is determined that projections were not present, was specified as a base portion, and the base friction was measured with TI-950 type TriboIndenter manufactured by Hysitron, Inc. by the method described above.

An SEM image of the same portion as the portion observed with the AFM image was observed to obtain a component map, and it was confirmed that the extracted projections having a height equal to or greater than 15 nm from the reference surface were projections formed of alumina or colloidal silica. In the examples and the comparative examples, in the component map obtained with the SEM, alumina and colloidal silica were not confirmed in the base portion.

(2) Thicknesses of Non-Magnetic Support and Each Layer

The thicknesses of the magnetic layer, the non-magnetic layer, the non-magnetic support, and the back coating layer of each manufactured magnetic tape were measured by the following method. As a result of the measurement, in all of the magnetic tapes, the thickness of the magnetic layer was 50 nm, the thickness of the non-magnetic layer was 0.7 μm, the thickness of the non-magnetic support was 5.0 μm, and the thickness of the back coating layer was 0.5 μm.

The thicknesses of the magnetic layer, the non-magnetic layer, and the non-magnetic support measured here were used for calculating the following refractive index.

(i) Manufacturing of Cross Section Observation Sample

A cross section observation sample including all regions of the magnetic tape from the magnetic layer side surface to the back coating layer side surface in the thickness direction was manufactured according to the method disclosed in paragraphs 0193 and 0194 of JP2016-177851A.

(ii) Thickness Measurement

The manufactured sample was observed with the STEM and a STEM image was captured. This STEM image was a STEM-high-angle annular dark field (HAADF) image which is captured at an acceleration voltage of 300 kV and a magnification ratio of imaging of 450,000, and the imaging was performed so that entire region of the magnetic tape from the magnetic layer side surface to the back coating layer side surface in the thickness direction in one image. In the STEM image obtained as described above, a linear line connecting both ends of a line segment showing the surface of the magnetic layer was determined as a reference line showing the surface of the magnetic tape on the magnetic layer side. In a case where the STEM image was captured so that the magnetic layer side of the cross section observation sample was positioned on the upper side of the image and the back coating layer side was positioned on the lower side, for example, the linear line connecting both ends of the line segment described above is a linear line connecting an intersection between a left side of the image (shape is a rectangular or square shape) of the STEM image and the line segment, and an intersection between a right side of the STEM image and the line segment to each other. In the same manner as described above, a reference line showing the interface between the magnetic layer and the non-magnetic layer, a reference line showing the interface between the non-magnetic layer and the non-magnetic, support, a reference line showing the interface between the non-magnetic support and the back coating layer, and a reference line showing the surface of the magnetic tape on the back coating layer side were determined.

The thickness of the magnetic layer was obtained as the shortest distance from one position randomly selected on the reference line showing the surface of the magnetic tape on the magnetic layer side to the reference line showing the interface between the magnetic layer and the non-magnetic layer. In the same manner as described above, the thicknesses of the non-magnetic layer, the non-magnetic support, and the back coating layer were obtained.

(3) ΔN of Magnetic Layer

Hereinafter, M-2000U manufactured by J. A. Woollam Co., Inc. was used as the ellipsometer. The creating and fitting of a double-layer model or a single-layer model were performed with WVASE32 manufactured by J. A. Woollam Co., Inc. as the analysis software.

(i) Measurement Refractive Index of Non-Magnetic Support

A sample for measurement was cut out from each magnetic tape. The cloth not used was permeated with methyl ethyl ketone, the back coating layer of the sample for measurement was wiped off and removed using this cloth to expose the surface of the non-magnetic support, and then, this surface is roughened with sand paper so that reflected light of the exposed surface is not detected in the measurement which will be performed after this with the ellipsometer.

After that, by causing the cloth to permeate with methyl ethyl ketone, by wiping off and removing the magnetic layer and the non-magnetic layer of the sample for measurement using the cloth and bonding a surface of a silicon wafer and the roughened surface to each other using static electricity, the sample for measurement was disposed on the silicon wafer so that the surface of the non-magnetic support exposed by removing the magnetic layer and the non-magnetic layer (hereinafter, referred to as the “surface of the non-magnetic support on the magnetic layer side”) faced upward.

The incidence ray was incident to the surface of the non-magnetic support of the sample for measurement on the magnetic layer side on the silicon wafer using the ellipsometer as described above, to measure Δ and ψ. By using the obtained measurement values and the thickness of the non-magnetic support obtained in the section (2), the refractive indexes of the non-magnetic support (the refractive index in a longitudinal direction, the refractive index in a width direction, the refractive index in a thickness direction measured by incidence of incidence ray in the longitudinal direction, and the refractive index in a thickness direction measured by incidence of incidence ray in the width direction) were obtained by the method described above.

(ii) Measurement of Refractive Index of Non-Magnetic Layer

A sample for measurement was cut out from each magnetic tape. The cloth not used was permeated with methyl ethyl ketone, the back coating layer of the sample for measurement was wiped off and removed using this cloth to expose the surface of the non-magnetic support, and then, this surface is roughened with sand paper so that reflected light of the exposed surface is not detected in the measurement which will be performed after this with the spectroscopic ellipsometer.

After that, the cloth not used was permeated with methyl ethyl ketone, the surface of the magnetic layer of the sample for measurement was wiped off using this cloth, the magnetic layer was removed to expose the surface of the non-magnetic layer, and then, the sample for measurement was disposed on the silicon wafer in the same manner as in the section (i).

The measurement regarding the surface of the non-magnetic layer of the sample for measurement on the silicon wafer was performed using the ellipsometer, and the refractive indexes of the non-magnetic layer (the refractive index in a longitudinal direction, the refractive index in a width direction, the refractive index in a thickness direction measured by incidence of incidence ray in the longitudinal direction, and the refractive index in a thickness direction measured by incidence of incidence ray in the width direction) were obtained by the method described above by spectral ellipsometry.

(iii) Measurement of Refractive Index of Magnetic Layer

A sample for measurement was cut out from each magnetic tape. The cloth not used was permeated with methyl ethyl ketone, the back coating layer of the sample for measurement was wiped off and removed using this cloth to expose the surface of the non-magnetic support, and then, this surface is roughened with sand paper so that reflected light of the exposed surface is not detected in the measurement which will be performed after this with the spectroscopic ellipsometer.

After that, the sample for measurement was disposed on the sample for measurement on the silicon wafer, in the same manner as in the section (i).

The measurement regarding the surface of the magnetic layer of the sample for measurement on the silicon wafer was performed using the ellipsometer, and the refractive indexes of the magnetic layer (the refractive index Nx in a longitudinal direction, the refractive index Ny in a width direction, the refractive index Nz₁ in a thickness direction measured by incidence of incidence ray in the longitudinal direction, and the refractive index Nz₂ in a thickness direction measured by incidence of incidence ray in the width direction) were obtained by the method described above by spectral ellipsometry. Nxy and Nz were obtained from the obtained values, and the absolute value ΔN of the difference of these values was obtained. Regarding all of magnetic tapes of the examples and the comparative examples, the obtained Nxy was a value greater than Nz (that is, Nxy>Nz).

(4) Vertical Squareness Ratio (SQ)

A vertical squareness ratio of the magnetic tape is a squareness ratio measured regarding the magnetic tape in a vertical direction. The “vertical direction” described regarding the squareness ratio is a direction orthogonal to the surface of the magnetic layer. Regarding each magnetic tape of the examples and the comparative examples, the vertical squareness ratio was obtained by sweeping an external magnetic field in the magnetic tape at a measurement temperature of 23° C.±1° C. using an vibrating sample magnetometer (manufactured by Toei Industry Co., Ltd.) under conditions of a maximum external magnetic field of 1194 kA/m (15 kOe) and a scan speed of 4.8 kA/m/sec (60 Oe/sec), The measurement value is a value after diamagnetic field correction, and is obtained as a value obtained by subtracting magnetization of a sample probe of the vibrating sample magnetometer as background noise. In one aspect, the vertical squareness ratio of the magnetic tape is preferably 0.60 to 1.00 and more preferably 0.65 to 1.00. In addition, in one aspect, the vertical squareness ratio of the magnetic tape can be, for example, equal to or smaller than 0.90, equal to or smaller than 0.85, or equal to or smaller than 0.80, and can also be greater than these values.

Missing Pulse Generation Frequency

The following evaluation was performed in the low temperature and high humidity environment of a temperature of 13° C. and relative humidity of 80%.

A magnetic tape cartridge accommodating each magnetic tape (magnetic tape total length of 500 m) of the examples and the comparative examples was set in a drive of Linear Tape-Open Generation 6 (LTO-G6) manufactured by IBM. Then, in the drive, the magnetic tape in the magnetic tape cartridge was subjected to reciprocating running 1,500 times at tension of 0.6 N and a running speed of 8 m/sec, while bringing a magnetic head of the drive and the surface of the magnetic layer into contact with each other and sliding.

The magnetic tape cartridge after the running was cut out from the drive, and set in another drive (LTO-G6 drive manufactured by IBM), and the recording and reproducing of information were performed, while allowing the magnetic tape to run and bringing the magnetic head and the surface of the magnetic layer into contact with each other and sliding. A reproducing signal during the running was introduced to an external analog/digital (AD) conversion device. A signal having a reproducing signal amplitude which is decreased by 70% or more than an average (average of measured values at each track) was set as a missing pulse, a generation frequency (number of times of the generation) thereof was divided by the total length of the magnetic tape to obtain a missing pulse generation frequency (unit: times/m) per unit length (per 1 m) of the magnetic tape. In a case where the missing pulse generation frequency is equal to or smaller than 5.0 times/m, the magnetic tape can be determined as a magnetic tape having high reliability in practice.

The results of the above evaluation are shown in Table 1 (Table 1-1 to Table 1-4).

TABLE 1 Example 1 Example 2 Example 3 Ferromagnetic Average particle 22 22 22 powder (1) size (nm) Amount 99.0% 99.0% 99.0% Ferromagnetic Average particle 60 60 60 powder (2) size (nm) Amount 1.0% 1.0% 1.0% Magnetic liquid bead dispersion time 50 hours 50 hours 50 hours Magnetic liquid dispersion bead diameter 0.1 mm 0.1 mm 0.1 mm Magnetic liquid 330 eq/ton 330 eq/ton 330 eq/ton Content of SO₃Na group of polyurethane resin Magnetic liquid 15.0 parts 15.0 parts 15.0 parts Content of SO₃Na group-containing polyurethane resin Non-magnetic layer forming composition dispersion time 24 hours 24 hours 24 hours Magnetic layer drying temperature 50° C. 50° C. 50° C. Calender temperature 100° C. 100° C. 100° C. Formation and alignment of magnetic layer Homeotropic Homeotropic Homeotropic alignment 0.5 T alignment 0.5 T alignment 0.5 T Result Vertical squareness ratio (SQ) 0.66 0.66 0.66 ΔN 0.30 0.30 0.30 Base friction 0.30 0.28 0.30 Missing pulse generation frequency (times/m) 3.0 3.0 2.0 Example 4 Example 5 Example 6 Ferromagnetic Average particle 22 22 22 powder (1) size (nm) Amount 99.0% 98.5% 99.0% Ferromagnetic Average particle 60 60 60 powder (2) size (nm) Amount 1.0% 1.5% 1.0% Magnetic liquid bead dispersion time 50 hours 50 hours 50 hours Magnetic liquid dispersion on bead diameter 0.1 mm 0.1 mm 0.1 mm Magnetic liquid 330 eq/ton 330 eq/ton 330 eq/ton Content of SO₃Na group of polyurethane resin Magnetic liquid 15.0 parts 15.0 parts 15.0 parts Content of SO₃Na group-containing polyurethane resin Non-magnetic layer forming composition dispersion time 24 hours 24 hours 24 hours Magnetic layer drying temperature 50° C. 50° C. 50° C. Calender temperature 100° C. 100° C. 100° C. Formation and alignment of magnetic layer Second magnetic layer: Second magnetic layer: Homeotropic homeotropic alignment 0.5 T/ homeotropic alignment 0.5 T/ alignment 0.2 T First magnetic layer: First magnetic layer: no alignment process no alignment process Result Vertical squareness ratio (SQ) 0.60 0.60 0.60 ΔN 0.35 0.35 0.25 Base friction 0.28 0.20 0.28 Missing pulse generation frequency (times/m) 2.0 1.0 3.0 Comparative Comparative Comparative Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Ferromagnetic Average particle 22 22 22 22 22 22 powder (1) size (nm) Amount 100% 100% 100% 99.0% 100% 99.0% Ferromagnetic Average particle — — — 60 — 60 powder (2) size (nm) Amount — — — 1.0% — 1.0% Magnetic liquid bead dispersion time 6 hours 50 hours 6 hours 6 hours 50 hours 96 hours Magnetic liquid dispersion bead 1.0 mm 0.1 mm 1.0 mm 1.0 mm 0.1 mm 0.1 mm diameter Magnetic liquid 60 eq/ton 330 eq/ton 60 eq/ton 60 eq/ton 330 eq/ton 330 eq/ton Content of SO₃Na group of polyurethane resin Magnetic liquid 25.0 parts 15.0 parts 25.0 parts 25.0 parts 15.0 parts 10.0 parts Content of SO₃Na group- containing polyurethane resin Non-magnetic layer forming 3 hours 24 hours 3 hours 3 hours 24 hours 48 hours composition dispersion time Magnetic layer drying temperature 70° C. 50° C. 70° C. 70° C. 50° C. 30° C. Calender temperature 80° C. 100° C. 80° C. 80° C. 100° C. 110° C. Formation and alignment of No alignment Homeotropic Homeotropic Homeotropic No alignment Homeotropic magnetic layer process alignment 0.5 T alignment 0.5 T alignment 0.5 T process alignment 0.5 T Result Vertical 0.50 0.66 0.55 0.55 0.53 0.80 squareness ratio (SQ) ΔN 0.10 0.30 0.20 0.20 0.20 0.45 Base friction 0.48 0.40 0.45 0.30 0.45 0.28 Missing pulse 10.0 10.0 10.0 8.0 10.0 7.0 generation frequency (times/m) Comparative Comparative Comparative Example 7 Example 8 Example 9 Ferromagnetic Average particle 22 22 22 powder (1) size (nm) Amount 100% 100% 100% Ferromagnetic Average particle — — — powder (2) size (nm) Amount — — — Magnetic liquid bead dispersion time 50 hours 96 hours 6 hours Magnetic liquid dispersion bead diameter 0.1 mm 0.1 mm 1.0 mm Magnetic liquid 330 eq/ton 330 eq/ton 60 eq/ton Content of SO₃Na group of polyurethane resin Magnetic liquid 15.0 parts 10.0 parts 25.0 parts Content of SO₃Na group-containing polyurethane resin Non-magnetic layer forming composition dispersion time 24 hours 48 hours 3 hours Magnetic layer drying temperature 50° C. 30° C. 70° C. Calender temperature 100° C. 110° C. 80° C. Formation and alignment of magnetic layer Second magnetic layer: Second magnetic layer: Second magnetic layer: no alignment process/ no alignment process/ homeotropic alignment 0.5 T/ First magnetic layer: First magnetic layer: First magnetic layer: homeotropic alignment 0.5 T homeotropic alignment 0.5 T no alignment process Result Vertical 0.60 0.66 0.53 squareness ratio (SQ) ΔN 0.20 0.20 0.20 Base friction 0.45 0.45 0.40 Missing pulse 10.0 12.0 9.0 generation frequency (times/m)

From the results shown in Table 1, in the magnetic tapes of Examples 1 to 6 in which ΔN and the base friction of the magnetic layer are respectively in the range described above, it is possible to confirm that the missing pulse generation frequency is low, even in a case where sliding between a surface of the magnetic tape (surface of the magnetic layer) and a magnetic head is repeated in the low temperature and high humidity environment, compared to the magnetic tapes of Comparative Examples 1 to 9.

In general, the squareness ratio is known as an index for a state of the ferromagnetic powder present in the magnetic layer. However, as shown in Table 1, even in a ease of the magnetic tapes having the same vertical squareness ratios, ΔN are different from each other (for example, Examples 1 to 3 and Comparative Example 8). The inventors have thought that this shows that ΔN is a value which is affected by other factors, in addition to the state of the ferromagnetic powder present in the magnetic layer.

One aspect of the invention is effective in a technical field of various magnetic tapes such as magnetic tapes for data storage. 

What is claimed is:
 1. A magnetic tape comprising: a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, wherein an absolute value ΔN of a difference between a refractive index Nxy measured regarding an in-plane direction of the magnetic layer and a refractive index Nz measured regarding a thickness direction of the magnetic layer is 0.25 to 0.40, and a coefficient of friction measured regarding a base portion of a surface of the magnetic layer is equal to or smaller than 0.30.
 2. The magnetic tape according to claim 1, wherein the difference Nxy−Nz between the refractive index Nxy and the refractive index Nz is 0.25 to 0.40.
 3. The magnetic tape according to claim 1, wherein the coefficient of friction measured regarding the base portion of the surface of the magnetic layer is 0.10 to 0.30.
 4. The magnetic tape according to claim 2, wherein the coefficient of friction measured regarding the base portion of the surface of the magnetic layer is 0.10 to 0.30.
 5. The magnetic tape according to claim 1, further comprising: a non-magnetic layer including non-magnetic powder and a binding agent between the non-magnetic support and the magnetic layer.
 6. The magnetic tape according to claim 1, further comprising: a back coating layer including non-magnetic powder and a binding agent on a surface of the non-magnetic support opposite to a surface provided with the magnetic layer.
 7. A magnetic tape cartridge, which comprises a magnetic tape comprising: a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, wherein an absolute value ΔN of a difference between a refractive index Nxy measured regarding an in-plane direction of the magnetic layer and a refractive index Nz measured regarding a thickness direction of the magnetic layer is 0.25 to 0.40, and a coefficient of friction measured regarding a base portion of a surface of the magnetic layer is equal to or smaller than 0.30.
 8. The magnetic tape cartridge according to claim 7, wherein the difference Nxy−Nz between the refractive index Nxy and the refractive index Nz is 0.25 to 0.40.
 9. The magnetic tape cartridge according to claim 7, wherein the coefficient of friction measured regarding the base portion of the surface of the magnetic layer is 0.10 to 0.30.
 10. The magnetic tape cartridge according to claim 8, wherein the coefficient of friction measured regarding the base portion of the surface of the magnetic layer is 0.10 to 0.30.
 11. The magnetic tape cartridge according to claim 7, wherein the magnetic tape further comprises a non-magnetic layer including non-magnetic powder and a binding agent between the non-magnetic support and the magnetic layer.
 12. The magnetic tape cartridge according to claim 7, wherein the magnetic tape further comprises a back coating layer including non-magnetic powder and a binding agent on a surface of the non-magnetic support opposite to a surface provided with the magnetic layer.
 13. A magnetic tape apparatus, which comprises: a magnetic head; and a magnetic tape comprising: a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, wherein an absolute value ΔN of a difference between a refractive index Nxy measured regarding an in-plane direction of the magnetic layer and a refractive index Nz measured regarding a thickness direction of the magnetic layer is 0.25 to 0.40, and a coefficient of friction measured regarding a base portion of a surface of the magnetic layer is equal to or smaller than 0.30.
 14. The magnetic tape apparatus according to claim 13, wherein the difference Nxy−Nz between the refractive index Nxy and the refractive index Nz is 0.25 to 0.40.
 15. The magnetic tape apparatus according to claim 13, wherein the coefficient of friction measured regarding the base portion of the surface of the magnetic layer is 0.10 to 0.30.
 16. The magnetic tape apparatus according to claim 14, wherein the coefficient of friction measured regarding the base portion of the surface of the magnetic layer is 0.10 to 0.30.
 17. The magnetic tape apparatus according to claim 13, wherein the magnetic tape further comprises a non-magnetic layer including non-magnetic powder and a binding agent between the non-magnetic support and the magnetic layer.
 18. The magnetic tape apparatus according to claim 13, wherein the magnetic tape further comprises a back coating layer including non-magnetic powder and a binding agent on a surface of the non-magnetic support opposite to a surface provided with the magnetic layer. 